Hydrocephalus is a condition associated with ventricular enlargement caused by net accumulation of fluid in the ventricles of the brain. Non-communicating hydrocephalus is hydrocephalus associated with an obstruction in the ventricular system and is generally characterized by increased cerebrospinal fluid (CSF) pressure. In contrast, communicating hydrocephalus is hydrocephalus associated with obstructive lesions within the subarachnoid space. Normal Pressure Hydrocephalus (NPH), a form of communicating hydrocephalus, primarily occurs in persons over 60 years of age and is characterized by CSF at normal pressure. Classic symptoms of NPH include gait disturbance, incontinence and dementia. In summary, NPH presents as an enlargement of the ventricles with a normal CSF pressure.
The objective in the treatment of hydrocephalus is to reduce the ventricular pressure so that ventricular size returns to a normal level. Hydrocephalus is often treated by implanting into the brain a shunt that drains excess CSF from the ventricles. These shunts are generally comprised of a cerebral catheter inserted through the brain into the ventricle and a one-way valve system that drains fluid from the ventricle into a reservoir of the body, such as the jugular vein or the peritoneal cavity. U.S. Pat. Nos. 3,886,948, 3,288,142 and 4,595,390 describe a shunt that has a spherical sapphire ball biased against a conical valve seat by stainless steel spring. The pressure of the CSF pushes against the sapphire ball and spring in the direction tending to raise the ball from the seat. When the pressure difference across the valve exceeds a so-called “popping” or opening pressure, the ball rises from the seat to allow CSF to flow through the valve and thereby vent CSF. U.S. Pat. No. 4,595,390 describes an externally programmable shunt valve that allows the pressure setting of the valve to be varied by applying a transmitter that emits a magnetic signal over the head of the patient over the location of the implanted shunt. Use of an external programmer with a magnetic transmitter allows the pressure setting of the valve to be adjusted according to the size of the ventricles, the CSF pressure and the treatment objectives.
Although magnetically adjustable shunts allow the pressure of an implanted shunt to be adjusted externally, these art-known shunts are associated with some limitations. For example, when a patient with an implanted magnetically adjustable shunt valve is within proximity of a strong magnet or strong magnetic field, such as a magnetic resonance imaging (MRI) device, the pressure setting of the valve can change. In addition, verification of the pressure setting of art-known magnetic valves can require use of a radiopaque marker on the valve that is detected using an X-ray taken of the location that the valve is implanted.
It would therefore be desirable to design improved ventricular shunts.